The present invention relates to apparatus for use with metallic structures. In particular, the present invention relates to apparatus, for use with metallic structures such as pipes, which facilitates corrosion protection and detection of structure location.
Metallic structures, such as pipes, cables and storage tanks which are buried underground are exposed to conditions which can promote corrosion of the structure. To try and minimise environmental damage to these structures, various methods of minimising corrosion have been developed. In particular, it is well known to use the technique of cathodic protection to reduce the occurrence of corrosion in such metallic structures.
To apply cathodic protection to a metallic structure, a protection system is used. An example of a pipe 12 provided with a known protection system 10 is shown in FIG. 1 of the accompanying drawings. The protection system 10 of FIG. 1 is an impressed current cathodic protection (ICCP) system in which an anode 14 which is external to the structure 12 is connected to a Transformer Rectifier unit 16. The Transformer Rectifier unit 16 is also connected with the structure 12, and applies a DC voltage (and hence a DC current) across the anode and the structure, so that the structure is at a negative potential with respect to the anode 14. Corrosion of the structure is reduced by virtue of the fact that the anode 14, the Transformer Rectifier unit 16 and the structure 12 form an electrochemical cell. The DC current that flows serves to reduce or stop corrosive anodic reactions occurring on the surface of the metallic structure 12. Such ICCP systems are well known.
It is well know that the protection application system can be provided with a feature which enables the DC cathodic protection current being applied to the structure to be switched on and off. By switching the DC current on and off in a controlled manner an identifiable pattern of interruptions in the current flow will be generated. The interruption of the DC current is performed using equipment such as an Interrupter device, relay or switch.
The interrupted DC current pattern can be used in various ways, for example, an interrupted DC current pattern can be used to determine the effectiveness of the cathodic protection by applying the interrupted signal to the structure, in this case a pipe. A measurement device is then connected directly to the pipe at a desired point along the pipe and potential measurements are taken with the current both on and off. These measurements are then repeated at other desired points along the pipe. The measurements are then used to determine that the normally applied cathodic protection current is at an acceptable level to prevent corrosion.
In addition, an interrupted DC current being applied to a pipe can be used to identify specific pipelines or the area of influence of a particular protection application system by making a connection to the pipe at a desired location and using a meter to measure the voltage and current patterns at that location. These measurements can then be analysed with reference to the applied interrupted DC current pattern to verify that the current is reaching the measurement point. For example, an Interrupter device could be set to apply a signal having a pattern of 1 second off and 4 seconds on. If a technician then connects a measurement device such as a voltmeter to a point on the pipeline and observes a change in potential consistent with this pattern then he knows that he is connected to the correct pipe and also that the protection application system does have an influence at this location.
An interrupted DC current can also be used to locate coating defects on the pipe by inserting detection probes into the soil above the pipe and measuring the current flowing in the soil. The absolute value of the current flowing in the soil is of little value but if the pattern matches that of the applied interrupted DC signal, the magnitude of the change (from on to off or vice versa) indicates the magnitude of the coating defect.
Remote monitoring and control of cathodic protection levels applied to structures, such as pipelines, is well documented and this can be performed using either wired connection methods, such as telephone lines or a computer network, or wireless connection methods such as radio, cellular modem or satellite communication.
To carry out the analysis of the area of protection on of an application system, or establish the location of a suspected default, it is necessary to know the actual location of the subterranean structure. Similarly, construction workers or mining and quarrying workers must know the exact location of buried structures such as pipes or cables to avoid causing accidental damage during excavation work. However, maps of underground pipes and cables are not always available or accurate. In view of this, a technique of locating and subsequently mapping underground structures by applying an AC signal to them and then using an electromagnetic system, in this case a pipe locator, to determine the physical location of the underground pipe has been developed. The AC signal is usually applied to the structure using a portable transmitter. To determine the location of a pipe, a portable transmitter is temporarily connected to the pipe either directly or by induction at a known location of the pipe. A pipe locator is then used to determine the pipe location in the surrounding area. However, the location of at least one local part of the pipe in the area must be known so that the portable transmitter can be connected with it to enable to remainder of the pipe location to be determined.